Bending arm Using armatures problem
Select the mesh that you want to deform and go to the object data properties menu:



Then you can change the viewport mode to weigth paint in the upper left corner:



Now you can select the bone how misbehaves in the bottom right under the vertex groups
and paint over it a weight of 1 means that the movement of the bone Influences the mesh 100 % and a weight of 0 means that it doesn't influences it.